The health of a prime mover, such as a pump, is a significant variable in high-reliability applications. The motor current signature of the electric motor driving a pump is a reliable indicator of the health of the electromechanical system, provided the signature is analyzed properly. In this general area known as electrical signature analysis (ESA), this technology includes special monitoring and processing of motor currents to determine characteristics of a motor, pump and its mechanical load. The motor takes electrical energy and converts it into mechanical energy to drive a mechanical load. Variations in the mechanical load and changes in the motor or pump condition are reflected in the motor current. By using special detection circuits and processing techniques, these small variations of motor current can be captured and analyzed. This information provides very useful and descriptive information about the conditions of the combined motor, pump and mechanical load.
The motor current contains motor current noise from various sources. It has been found that the motor current noise includes the sum of all the mechanical load changes which refer back to the electric motor drive and pump. Thus, motor current noise signatures taken at different periods during the operating life of the device help determine aging and wear or abnormal operating characteristics. The relative magnitude of the electric noise signal generated by a particular mechanical noise source will depend on its absolute magnitude and on its mechanical linkage to the motor which remains a fixed relationship for a given device. The motor itself acts as a transducer changing the mechanical load variations into electrical noise.
Currently, there is no easy, human friendly way to quickly check the status of a motor, pump and load using ESA. Typically, an ESA analysis requires examination of time and frequency domain plots. The equipment associated with such an analysis is not only bulky, expensive and time-consuming to use, but makes it virtually impossible to run a quick “health check” of a particular motor and load.
Furthermore, given the large amount of information held within the motor current noise signals, there is a need to provide high performance filtering circuits to improve ESA discrimination.
In general, some systems utilizing multiple motors do not have a stiff voltage source that is capable of maintaining a constant voltage irrespective of variations in any one of the motors. As a result, motor current signature analysis may identify a problem with a particular motor when in fact the problem arises from another motor in the same system. Consequently, a motor can be detected as having a broken bar or other defect, be taken off line, and then, on physical examination, be found to not have a defect. Signals representative of such false defect detections unpredictably appear and disappear and are sometimes referred to as “ghosts”. These false indications of defects are often caused by the spurious signals generated in a typical weak voltage system such as may be found on aircraft, ships, locomotives or other vehicles utilizing on-board generated power. Accordingly, it would be advantageous to provide a device and method which enables identification and removal of motor current anomalies which are caused by spurious or ghost signals on the power system and not by fault conditions associated with the motor.
U.S. Pat. No. 5,578,937 to Haynes et al., herein incorporated by reference, teaches a method for diagnosing induction motors that requires a voltage signal and has no qualitative diagnostics indicative of the health of a prime mover (pump) associated with the induction motor.
U.S. Pat. No. 5,689,194 to Richards et al., herein incorporated by reference, teaches a motor current signature analysis system using a qualitative audio listening section and computer controlled frequency shifting/filtering with no qualitative diagnostics indicative of the health of a prime mover (pump) associated with the motor.
The publication, “Electrical Signature Analysis (ESA) As A Diagnostic Maintenance Technique for Detecting The High Consequence Fuel Pump Failure Modes”; D. E. Welch, H. D. Haynes, D. F. Cox, R. J. Moses; 6th Joint FAA/DoD/NASA Conference on Aging Aircraft, Sep. 16, 2002, herein incorporated by reference, provides more details on early work on this invention. More recent work on the invention resulted in additional references included herein.